The rationale of this series of experiments is that aging constitutes a progressive and irreversible lesion of the SCN and associated neuronal systems critical to circadian timekeeping. A small but convincing body of evidence suggests that there is gradual cell loss in the SCN with age. Furthermore, there is a corresponding decrease in rhythm amplitude in light-dark cycles (although not necessarily in constant conditions) and rhythm period in constant conditions. There is also evidence for an age- related deterioration in critical neuronal systems afferent to the SCN, and perhaps in certain systems efferent to the SCN. Unavoidably, most of this data was gathered in cross-sectional studies. Anatomical, physiological and behavioral correlates of aging in the SCN have never been simultaneously studied in the strongest possible paradigm, the longitudinal study. A number of mathematical models explicitly predict the relation between cell number in the SCN and various characteristics of circadian rhythmicity (period, amplitude, phase). A detailed understanding of this relationship and of the necessary cellular substrate of rhythmicity may be gained through a combination of cross- sectional and longitudinal studies across the lifespan of the rodent. We will specifically determine whether age-related deficits occur in major systems afferent to the SCN by measuring in vivo release of neurotransmitters specific to the retinohypothalamic tract (RHT), geniculohypothalamic tract (GHT), and the raphe-hypothalamic projection across the rat lifespan. We will determine whether age-related deficits occur in the intrinsic circuitry of the biological clock through examination of 1) circadian rhythms in single unit activity in the suprachiasmatic nucleus (SCN) in vitro, 2) the ability of neurotransmitters and second messengers to phase shift such rhythms in vitro, 3) basal and stimulated release of neurotransmitters in the SCN in vitro, 4) age-related loss of immunohistochemically-identified cell types in the SCN. Finally, we will determine whether potassium stimulation of the SCN in vivo produces smaller behavioral phase shifts in old rats than in young rats. In conjunction with studies of intrinsic SCN mechanisms, this study will allow us to determine whether mechanisms efferent to the SCN degenerate with age. These studies will be conducted in three age groups initially (young = 4-6 mo., middle-aged = 18-22 mo., old = 28-32 mo.), with subsequent finer temporal grain studies of deficits observed in the initial studies. We believe these studies will in large part characterize the specific effects of aging on the biological clock, while providing knowledge necessary to eventual therapeutic intervention in age-related circadian disorders (sleep fragmentation, insomnia, some forms of depression).